In known air classification systems for receiving mixed materials and separating them into respective groups of light and heavy materials there is employed a rotary drum air classifier. The materials being separated, such as municipal waste for example, may include only items of a dry nature but often include such items as garbage or the like which contains considerable amounts of water.
The classifying apparatus includes conveyor means for depositing the mixed unclassified materials within the drum at a selected point along the length of the drum. The drum is inclined at an angle, such as 10.degree. for example, to the horizontal and means is provided for rotating the drum about its longitudinal axis. The materials are deposited in the drum while the drum is being rotated and a stream of air is simultaneously directed at relatively high velocity into the lower end of the drum so as to entrain light materials and carry them out the upper end of the drum to additional processing apparatus.
The drum is provided internally with longitudinally extending vanes or lifters which, as the drum rotates, causes the heavy materials to be continuously raised and dropped within the drum so that, because of the inclination of the drum, the heavy materials will eventually work their way toward and out the lower end of the drum in the known manner. It will be apparent that light materials not initially entrained in the air stream when the materials fall from the load conveyor will be eventually separated from the heavy materials as the tumbling process continues.
It is known that the inclusion of any substantial amounts of moisture in the materials not only will cause sticking together of materials and adhesion of wet materials to the walls of the drum, resulting in possible jamming and clogging of the apparatus, but also will cause some possibly substantial amounts of light materials to be separated out as heavy materials because of the added weight caused by the water content. Separate driers for removing the moisture from the materials before they are deposited in the drum is a conventional manner of correcting the situation. However, this entails added expense and the addition of undesirable space-consuming apparatus into the system.